Introduction

The traditional graphics subsystem in R is very flexible when it comes to producing standard statistical graphics. It provides a collection of high-level plotting functions that produce entire coherent displays, several low-level routines to enhance such displays and provide finer control over the various elements that make them up, and a system of parameters that allows global control over defaults and other details. However, this system is not very proficient at combining multiple plots in a page. It is quite straightforward to produce such plots; however, doing so in an effective manner, with properly coordinated scales, aspect ratios, and labels, is a fairly complex task that is difficult even for the experienced R user. Trellis graphics, originally implemented in S, was designed to address this shortcoming. The lattice add-on package provides similar capabilities for R users.

The name “Trellis” comes from the trellislike rectangular array of panels of which such displays often consist. Although Trellis graphics is typically associated with multiple panels, it is also possible to create single-panel Trellis displays, which look very much like traditional high-level R plots. There are subtle differences, however, mostly stemming from an important design goal of Trellis graphics, namely, to make optimum use of the available display area. Even single-panel Trellis displays are usually as good, if not better, than their traditional counterparts in terms of default choices. Overall, Trellis graphics is intended to be a more mature substitute for traditional statistical graphics in R. As such, this book assumes no prior knowledge of traditional R graphics; in fact, too much familiarity with it can be a hindrance, as some basic assumptions that are part and parcel of traditional R graphics may have to be unlearned. However, there are many parallels between the two: both provide high-level functions to produce comprehensive statistical graphs, both provide fine control over annotation and tools to augment displays, and both employ a system of user-modifiable global parameters that control the details of the display. This chapter gives a preview of Trellis graphics using a few examples; details follow in later chapters.